In order to increase the energy density and output of nonaqueous electrolyte secondary batteries, a study on using, as a negative electrode active material, a material that forms an alloy with lithium, such as silicon, germanium, tin, or zinc, instead of a carbon material such as graphite has been conducted. However, a negative electrode that uses a material containing silicon or the like as a negative electrode active material undergoes considerable volume expansion or shrinkage during occlusion and release of lithium. Therefore, in nonaqueous electrolyte secondary batteries including a negative electrode that uses a material containing silicon as a negative electrode active material, swelling of cells, formation of fine powder of a negative electrode active material, and detachment of a negative electrode active material from a current collector by stress occur as the charge-discharge cycle proceeds, which results in degradation of cycle characteristics.
PTL 1 below discloses a nonaqueous electrolyte secondary battery that uses a negative electrode obtained by forming a plurality of pillar-shaped protruding portions on a thin film that is made of a negative electrode active material such as silicon and deposited on a negative electrode current collector. The plurality of pillar-shaped protruding portions are made of a negative electrode active material such as silicon and have a larger thickness than portions around the protruding portions.
The negative electrode in the nonaqueous electrolyte secondary battery disclosed in PTL 1 below is obtained by forming a silicon thin film serving as a base layer on a surface of a negative electrode current collector by a sputtering method and furthermore forming pillar-shaped protruding portions made of silicon on the surface of the silicon thin film by a lift-off method including sputtering and etching in a combined manner. The negative electrode has cavities that absorb the volume expansion of the negative electrode active material during charging and discharging around the pillar-shaped protruding portions, whereby the swelling of cells is suppressed and a large stress is prevented from being applied to the negative electrode current collector.